Chalking-resistant, calcined kaolin clay pigment and method of making

ABSTRACT

Novel pigments useful as primary extenders in exterior grade latex formulations are described. The pigments are composed of particles of calcined clay, substantially all the particles of which are finer than about 44 microns and having an average particle size, equivalent spherical diameter, within the range of about 3 to 10 microns, most preferably within the range of about 4 to 8 microns. The novel pigments are further characterized by having a significantly lower oil absorption value (ASTM) than conventional, calcined clay pigments. Exterior grade paint formulations containing such pigments can be formulated below the critical pigment volume concentration without substantially reducing opacity or chalking resistance.

This application is a division of application Ser. No. 326,630 filed12/2/81, now U.S. Pat. No. 4,427,450.

BACKGROUND OF THE INVENTION

This invention relates to novel chalking-resistant pigments, to theirmanufacture and to their use as opacifying extenders in exterior gradepaint formulations. More specifically the invention relates to novelcalcined clay pigments, the pigments being characterized by possessing aunique combination of coarse particle size and low oil absorption value.

The paint industry supplies consumer-oriented products of the solventand emulsion types. Solvent paints are relatively simple systems, easyto formulate but difficult for the consumer to use. Solvent paintscontain a binder (oil or resin), a solvent (thinner), drying agents andpigments. Emulsion or so-called "latex" paints are complex mixturescontaining surfactants, protective colloids, biocides, freeze-thawstabilizers, emulsifiers and water in addition to the one or more typesof pigment which may be used. Following their introduction after WorldWar II, latex paints have gained substantially in market acceptance.They now account for a majority of interior and exterior paint tradesales.

Interior and exterior latex paints have generally similar formulations.An important distinction however is that exterior grade paints containrelatively more binder and prime pigment but less extender pigment thaninterior paints. That is because paint film integrity and overalldurability are more critical in exterior paints than in interior grades.

An important parameter in paint formulation is the pigment volumeconcentration, or PVC. PVC is a control factor in the design of paintformulations, because paint properties are governed by volume ratherthan weight effects. The following equation defines the PVC as apercentage of volume of dried paint film: ##EQU1##

The critical pigment volume concentration, or CPVC, is defined as thatPVC at which air interfaces are generated in the dry paint film due todeficiency of binder with respect to pigment. It is well known that manypaint volume properties change drastically at CPVC. The relationshipbetween PVC and CPVC is nonlinear. There is authority for the view thatdifferent paints are properly compared on the basis of equal reducedpigment volume concentration, or RPVC. The RPVC is defined by thefollowing:

    RPVC=PVC/CPVC

Generally exterior grade paints have an RPVC less than 1 and interiorpaints have an RPVC greater than 1. There is considerable controversyover whether in latex paint the CPVC is a characteristic of the pigmentor a characteristic of both pigment and binder. There is publishedauthority for both points of view.

CPVC is related inversely to the amount of binder that the pigmentparticles "absorb". A conventionally used technique to determine thisproperty of a pigment or extender is the amount of linseed oil needed toform a paste containing a given weight of pigment. This is referred toin the art as oil absorption. As used herein the term "oil absorption"refers to the procedure described in ASTM D-281. Substitution of anequal amount of high oil absorption extender pigment for one of low oilabsorption results in a reduction of the CPVC of that paint. This inturn restricts the range of PVC that can be utilized in exteriorformulations and the amount of extender pigments which can be employed.

Exterior grade latex paints contain a mixture of prime and extenderpigments with titanium dioxide most generally used as the prime pigmentbecause of its outstanding optical properties. Zinc oxide is employed toa smaller extent. The most commonly used extender pigments for exteriorgrade latex paints are calcium carbonate and talc. Kaolin clays arerarely used in exterior grade paint formulations, and when they areemployed only small amounts are used for reasons which will be discussedbelow.

The binder in emulsion paints consists of globules (0.1 to 1.0 microndiameter) of film-forming polymer of 10,000 to 1,000,000 moleculeweight. The latex particle size and composition are varied to effectchanges in such properties as durability, gloss, glass transitiontemperature and the like. At present acrylic and vinyl-acrylic resinsaccount for the majority of binders used in latex paints.

The weatherability of exterior grade coatings is determined by theability of the coatings to resist chalking, fading and brittlement,gloss reduction, frosting and bleeding. Chalking, which is manifested bythe formation of a powder on a painted surface, is one of the mostundesirable performance characteristics of a paint. It involves thechemical degradation of the paint binder by atmospheric andmeteorological attack, from which loose, removable powder (the pigment)is evolved from the paint film at or just beneath the surface. Twodistinct mechanisms are believed to be responsible for chalking. Oneinvolves direct ultraviolet degradation of the binder. It is related tothe ultraviolet stabilizer of the binder. With present-day use ofultraviolet screens in paints this is no longer much of a problem. Thesecond mechanism occurs when a pigment acts as a catalyst for chemicaloxidation of the binder. Thus it is evident that chalking is acharacteristic of a paint film and the terms "chalking pigments" and"chalking-resistant pigments" as used herein will refer to chalking ofthe paint film containing pigment. Chalking is a problem stillconsidered substantially unsolved by those in the paint industry, andmethods for reducing chalking are continually sought by the paintindustry and its suppliers.

Extender pigments profoundly influence the properties of latex paints.They control texture, optical and flow properties. Extenders involve alarge group of materials with diverse chemical properties. In mostpublished studies of extenders, the pigments have been evaluated interms of PVC rather than RPVC. This makes quantitative comparisons ofextenders difficult. Kaolin extender pigments are widely used ininterior formulations, whereas the undesirable weathering properties ofkaolin clay pigments have severely limited their use in exterior gradepaint formulations. Several hypotheses have been suggested to describechalking by TiO₂, but little or no work has been done with extenderpigments.

Commercially available, pigment-grade hydrous kaolins have oilabsorption generally in the range 25-40 grams oil per 100 grams clay.Hydrous kaolins having the lower values in this range are desirable inexterior grade latex formulations but their chalking properties precludesuch use. The high oil absorption values resulting from conventionalcalcination of kaolin clay pigments, which is typically in the range of45-60 grams oil per 100 grams clay, preclude their use as primeextenders in latex paints formulated below CPVC.

An extensive study of simple extender pigment in vinyl polymer at 50%PVC is described by F. Liberti, Official Digest, vol. 33, March 1961,page 390. Liberti found that four particle sizes of talc, coarse (ASP®400) and fine (ASP 100) hydrous kaolin, calcined kaolin (nonspecifiedparticle sizes) and fine calcium carbonate all chalked at essentiallythe same rate, that is, these pigments all rated "fair" on a scale ofgood-fair-poor. Liberti also found that coarse calcium carbonate, ratedas "good", chalked less than the fine calcium carbonate, but the RPVCwas not controlled.

Kaolin clay pigments are supplied as pigments and extenders inuncalcined (hydrous) grades and calcined grades, the latter beingfavored where opacification (hiding power) is an important criterion.The hydrous grades include products composed predominantly of relativelyfine and relatively coarse particles and are frequently supplied asblends of fractions of different particle sizes. Hydrous grades thatcontain an appreciable content of particles larger than 2 microns asdetermined by sedimentation include a significant quantity of naturallyoccurring stacks or booklets as well as the individual platelets knownto characterize particles of kaolin that are finer than about 2 microns.The finer grades are composed predominantly of such individualplatelets. Delaminated hydrous kaolins are produced by mechanicallyaltering the naturally occurring stacks or booklets in whole(unfractionated) crude clays or coarse particle size fractions thereof.The delaminated grades generally have higher oil absorption values thannaturally occurring clay of similar particle size distribution asdetermined by sedimentation. Most commercially available grades ofcalcined kaolin pigments have average particle sizes in the range ofabout 1.0 to 3 microns, and oil absorption values above 40 g./100 g.Generally oil absorption increases inversely with average particle size.Ultrafine grades of calcined clay having an average particle size below1 micron are used as extenders and coating pigments by the paperindustry. Oil absorption values exceed 80 g. oil/100 g. clay. Coarseparticle size calcined clay pigments have been supplied by the industryas products having average particle sizes in the range of 4 to 7 micronswith oil absorption values above 45 g./100 g. A process for preparingsuch products involving stage-wise fractionation of a dispersed pulp ofcrude clay to selectively reject undersized and oversized particles isdescribed in U.S. Pat. No. 2,928,751 to Lyons. The calcined coarsekaolin pigment, as disclosed in this patent, is contemplated for use asan ingredient in the manufacture of ceramics.

U.S. Pat. No. 3,519,453 to Morris et al discloses blending calcined,delaminated clay with hydrous coating clays for coating applications inpaper. Calcination is shown to increase oil absorption and coarsenparticle size, mostly by agglomeration of fine material. U.S. Pat. No.3,403,041 discloses the use of a chemically modified calcinedmechanically delaminated clay in a paint formulated for interior use.U.S. Pat. No. 3,171,718 discloses (column 11) alkyd paint formulationscontaining calcined delaminated clay at 50% and 60% PVC. These paintsare also formulated for interior use.

While hydrous and calcined clays have enjoyed widespread use asextenders for interior paints, to the best of my knowledge known formsof kaolin pigments or extenders, both hydrous and calcined grades, havenot been used extensively by the paint industry as the principalextender in the formulation of exterior grade paints. Furthermore, tothe best of my knowledge known forms of kaolin clay do not possess thecombination of properties required for such use. These propertiesinclude adequate resistance to chalking, ability to be formulated at thehigh PVC necessary in exterior grade latex paint formulation andacceptable opacification properties.

It is known in the art that wet or dry milling of pulverized calcinedclay pigments serve to breakup agglomerates in the calcined clay. Thisreduces oil absorption but has been associated with a significant lossof opacifying ability when the calcined clay pigments were evaluated intheir intended end uses, namely in paper and in interior grade paints.Thus, post-milling has been a step avoided by suppliers of calcined claypigments designed for prior uses of such clays .

THE INVENTION

The present invention results form a series of findings relative tothose properties of clay pigments that have heretofore placed aconstraint upon their utility in exterior grade paint formulation. Ihave discovered that contrary to the prior art knowledge of the adverseeffect of milling on the opacifying properties of calcined clay pigmentsfor most uses, milling of calcined clay particles, in particular coarseparticles of calcined clay, to an extent sufficient to reducesubstantially the oil absorption thereof, can be practiced to permitadequate loading of the calcined clay without substantial impairment ofthe opacifying properties of the clay when the calcined and then milledclay is formulated below CPVC in exterior grade paint formulations. Ihave also discovered that such calcined and milled clay can be used asan extender pigment, preferably the principal extender in exterior gradepaint formulations, to produce paint films having chalk resistancecomparable to heretofore used talc and calcium carbonate extenders andsuperior hiding power.

In its broadest aspect novel calcined clay pigments of the presentinvention have an average particle size of 3 microns or above,preferably in the range 3 to 10 microns, and an oil absorption valuemore characteristic of hydrous kaolin, for example about 30 g. oil/100g. clay or below, than typical calcined kaolin. The pigment may beproduced by calcining a coarse particle size fraction of hydrous kaolinclay crude and subjecting the calcined material to controlled milling.

In a presently preferred embodiment of the invention calcined kaolinclay extender pigments of further reduced oil absorption value areproduced by blending a minor amount of a fine fraction of hydrous kaolinclay of average particle size below about 1 micron with a coarsefraction of hydrous kaolin clay of average particle size 3 to 10microns, the major component of the blend being the coarse fraction,calcining the blend and milling the calcined blend, whereby mechanicalproblems associated with milling fine calcined clay are minimized. Theparticle sizes of coarse and fine fractions of kaolin particles and theproportions thereof are such that the blended, calcined and milledproduct has an average particle size of about 3 microns or above,preferably in the range 3 to 10 microns. It will be recognized that thescope of the invention also includes calcining coarse and fine fractionsseparately, then blending and milling, or calcining and millingseparately then blending.

In practice of the invention milling is terminated before the particlesof calcined clay are so reduced in particle size as to create a fineparticle size calcined clay having an average particle size appreciablybelow 3 microns; for example 2-21/2 microns average particle size. Suchpigments will lack chalk resistance achievable when milled coarsercalcined clay particles are utilized.

Pigments of the invention may be used to extend a titania of otherprimary pigment in latex or solvent paint formulations. The pigments ofthe present invention are capable of being incorporated in exteriorgrade paints at higher loadings than previously attainable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Crude kaolin clays useful in the production of pigments of the inventionare preferably those that contain a substantial proportion of particles(clay booklets) larger than 2 microns e.s.d. The coarse clay particlesare concentrated from such crude clay by blunging the crude in water,degritting the resulting pulp, centrifuging the degritted clay toconcentrate one or more coarse size clay fractions as the centrifugeunderflow and, if necessary, repeating the centrifugation one or moretimes to remove fine particles as an overflow and oversize, e.g., plus20 micron particles. The recovered coarse size fraction of hydrous clayshould have an average particle size in the range of about 3 to 10microns, preferably in the range of about 4 to 8 microns. Particle sizeof the hydrous clay calciner feed is reflected in the particle size ofthe calcined and pulverized clay. Since coarse calcined clay has greaterchalk resistance than finer calcined clay, it is desirable to producefrom the crude a fraction that has maximum average particle size.However, calcined clay particles that are too large, e.g., larger than10 microns generally lack the tinting power of finer particles. Clayfractions having an average particle size in the range of about 4 to 8microns, e.s.d., strike a balance between these considerations.Generally, hydrous clay particles larger than about 20 microns should beavoided or at least should be present in minimal amount.

Those knowledgeable about the nature of crude clays and clay fractionswill recognize that naturally occurring clays are polydisperse. In otherwords, they are composed of particles of varying sizes. The same is trueof coarse size fractions of clays recovered from clay crude inconventional centrifuges. For example a coarse size fraction of kaolinclay having an average particle size of 4.5 microns may contain 90% byweight of particles finer than 14 microns, 70% by weight of particlesfiner than 7 microns and 25% by weight finer than 2 microns.

All particle sizes used herein refer to values obtained by conventionalwet sedimentation techniques using 2.58 as the value for particledensity in the case of hydrous clays and 2.63 as the value for particledensity for calcined clays. These values are conventionally expressed as"equivalent spherical diameter" or "e.s.d.". Average particle size, asused herein, refers to that value of particle size at which 50% byweight of the material is finer than and 50% is coarser than thedesignated value.

The coarse size fraction of the kaolin clay crude should not bemechanically delaminated prior to calcination. Among other effects,delamination will result in undesirable increase in oil absorption anddecrease in particle size.

The coarse size fraction of clay recovered as the underflow pulp fromthe centrifuges may be flocculated by addition of acid and/or alum priorto bleaching when using any conventional filtration apparatus such as arotary vacuum filter. Alternatively the pulp can be dewatered withoutflocculation in an electrically augmented vacuum filter. The filter cakemay be washed and then dried in conventional drying equipment such as arotary dryer. Alternatively the filter cake can be dispersed by additionof a suitable dispersant such as a condensed phosphate salt, sodiumsalt, an organic dispersant or ammonia, and then spray dried. Afterdrying by any of these techniques the dry clay should be pulverizedbefore the calcination step. Calcination occurs under conditions of timeand temperature such that the kaolin particles are substantiallydehydrated by passing through the characteristic endotherm--about 550°C. Milling, understood herein to mean reduction of particle size andgrinding, of the calcined "metakaolin" product follows calcination. Ifdesired, calcination may also be carried out such that the kaolin passesthrough the characteristic exotherm at about 980° C. Kaolin pigments sotreated are generally brighter but also more abrasive.

It is especially preferred, however, to use acid kaolin as the calcinerfeed. The salt content in the clay should be kept low, as the presenceof significant quantities will cause sintering during calcination andundesirably large increases in oil absorption.

The calcined product has a much higher oil absorption than the hydrouskaolin feed material. Milling reduces the oil absorption and may becontinued until oil absorption of the calcined kaolin has been loweredenough to provide an improved pigment, which when formulated below CPVCin exterior paints has low-chalking properties without significantlyreduced opacifying power. Generally it is preferred to have the oilabsorption below about 35 g. oil/100 g. clay, or in the range 10-30 g.oil/100 g. clay, in which most extender pigments fall. Milling equipmentin which compression, densifying or compacting forces predominate ispreferred, since the compressive-type forces produced in such a millappear to assist in obtaining optimum packing and low oil absorption.Ring-roller and muller are examples of preferred types of millingequipment.

Achieving low oil absorption with pigments of the present invention is afunction of type of mill, time of milling and choice of particle size inthe blend. On commercial scale equipment achievement of very low oilabsorption through milling can become prohibitively expensive. In thelaboratory values as low as about 20 g. oil/100 g. clay have beenachieved with some pigments of the present invention, and thus itappears obtaining oil absorption values below 20 g./g. may be difficultto achieve commercially with pigments of the present invention.

Fine particle size calcined clay may be obtained by fractionating thesame or a different crude clay used as a source of the coarse clay.Known centrifugation techniques are used to recover from a dispersedaqueous pulp of degritted hydrous kaolin clay--a fine particle fractionthat is at least 70% by weight, and preferably at least 80% by weight,and most preferably at least 90% by weight finer than 2 microns. Atypical fine particle size cut is 100% finer than 10 microns, 90% finerthan 2 microns, 70% finer than 1 micron, and 50% finer than 0.6 microns.Therefore particle size fractions can be blended as an aqueous pulp withthe coarse fraction before filtration and drying or the blending cantake place after calcination and before milling or after milling.Blending with fines, when desired, may be carried out by techniquesconventional in the art. The hydrous clay, for example, may be blendedin slurry form as various cuts exit from fractionating devices. Finematerial may be blended with coarse material in ratios up to 50% byweight of fine material. The preferred ratios will depend to a certaindegree on the average particle sizes of the coarse and fine cuts. Forcoarse cuts of average particle size 6-8 microns, the preferred blendingratio is in the range of 20-30% fine fraction by weight, the finefraction having an average particle size between 0.3 and 0.9 microns,and the remainder coarse fraction. When blending, especially goodresults are achieved when the amount of material in the coarse cut ofparticle size smaller than 2 microns is kept below about 10% by weightof the coarse cut.

Pigments of the invention may be used in latex or solvent paints withoutdeparting from conventional formulations or formulation techniques. Thepigment may be used as an extender in conjunction with titania or otherprimary pigment. A significant advantage of the pigment of the presentinvention is that, relative to other common extenders, it may be used toreplace more of the very expensive titania primary pigment in commonformulations without decreasing chalking resistance or opacity.

Chalking performance of pigments is commonly measured by direct exposureto atmospheric conditions on so-called "test-fences" situated in variouslocations. These tests take several years to produce results. In orderto produce results in a shorter time, accelerated weathering tests havebeen devised. A panel, coated with the paint formulation containing thepigment to be tested, may be exposed to ultraviolet radiation, highhumidity and high temperature in a UVCON® test chamber, available fromthe Atlas Electric Corporation. Testing procedures and conditions followASTM G53-77, in which the instrument is also described. After periods ofexposure a chalking rating may be determined qualitatively by visualcomparison with standard samples. A rating of 1 is poorest relative tochalking and a rating of 10 is best (lowest chalking), the ratingprocedure following ASTM D659-74. Weight loss methods may also be used.Occasionally the accelerated tests do not correlate as accurately asmight be desired with actual test fence data, and paint manufacturersoften use both tests. The accelerated test has the great advantage ofrequiring only about two months, not two or more years, for results.

In order to more fully illustrate the nature of the invention, thefollowing examples, not to be construed as limiting are presented:

EXAMPLE 1

A coarse-grade hydrous kaolin clay having a particle size distributionof 100% finer than 44 microns, 90% finer than 14 microns, 70% finer than7 microns, 50% finer than 4.1 microns, 33% finer than 2 microns, 24%finer than 1 micron and 10% finer than 0.4 microns was used as theparent material for obtaining a coarse-fraction hydrous kaolin clay forfurther processing. The coarse-grade clay had been produced by blunginga Georgia crude clay, degritting the crude and fractionating thedegritted crude by standard techniques in the clay art. Thisconventionally processed clay, an acid kaolin, was found to have veryminor amounts of residual soluble salts, for example less than 0.1%weight.

Two hundred and fifty grams (250 g.) of this clay was calcined in afixed bed in a muffle furnace at 450° C. for 90 minutes. After thiscalcination treatment the sample had an oil absorption value of 38 g.oil/100 g. clay by ASTM. The sample had lost 7% of its crystalline wateras measured by loss on ignition before and after calcination. The samplewas then milled in a mortar and pestle until an oil absorption value of30 g./100 g. was reached. It was then micropulverized through a0.020-inch screen to "fluff-up" the pigment which aids in dispersing thepigment in paint formulation processes. The calcined and milled producthad a particle size distribution of 100% finer than 30 microns, 90%finer than 14 microns, 69% finer than 7 microns and 50% finer than 4.6microns, 26% finer than 2% microns and 15% finer than 1 micron.

This and subsequent samples were used as sole pigments in latex paintformulations, described in another example following, for the purpose ofrating chalking characteristics of the pigments. The oil absorptionvalues obtained herein by following the ASTM procedure were reproducibleto within ±2 units for oil absorption numbers up to about 46 g. oil/100g. clay.

EXAMPLE 2

Same as Example 1, except calcination was accomplished at 600° C. for 2hours. Under these conditions the sample lost 92% of its crystallinewater as measured by L.O.I. before and after calcination. The oilabsorption value after calcination was 41 g. oil/100 g. clay, and aftermilling it was 30 g./100 g. Particle size distribution of the finalproduct was the same as in Example 1.

EXAMPLE 3

Same as Example 1, except calcination was accomplished at 900° C. for 2hours. Under these conditions the sample lost 98% of its crystallinewater as measured by L.O.I. before and after calcination. The oilabsorption value after calcination was 46 g. oil/100 g. clay, and aftermilling it was 30 g./100 g. Particle size distribution of the finalproduct was the same as in Example 1.

EXAMPLE 4

A sample of the coarse-grade hydrous kaolin of Example 1 was acidflocced and washed with water to remove virtually all traces of solublesalts, and air dried prior to calcination in a muffle furnace.Two-hundred and fifty grams (250 g.) was calcined in a laboratory mufflefurnace at 900° C. for 1 hour. This calcined clay had an oil absorptionof 37 g. oil/100 g. clay and was milled in a Lancaster Mixer-Muller,Type PC, for 1 hour, after which treatment of the oil absorption fell to31 g./100 g. The sample was then micropulverized through a 0.020-inchscreen. Particle size distribution of the calcined and milled productwas the same as in Example 1.

EXAMPLE 5

From the coarse-grade, hydrous kaolin clay of Example 1, i.e. the"parent" clay having an average particle size of 4.1 microns, a coarserfraction of average particle size 6.4 microns was obtained byacid-washing the coarse-grade clay to remove any dispersant thereon,water washing and then re-dispersing the clay in water containing sodiumhydroxide as the only dispersant. The clay was then fractionated bysedimentation techniques familiar to those in the art. The coarsefraction of the parent material was then dried and stored for use. Theparticle size distribution of the coarse fraction of the parent materialwas found by sedimentation techniques (Sedigraph) to be 100% finer than40 microns, 90% finer than 15 microns, 70% finer than 9.2 microns, 50%finer than 6.4 microns, 30% finer than 4.6 microns, 10% finer than 3microns and 5% finer than 2 microns.

For blending purposes a fine-grade hydrous kaolin clay having a particlesize distribution of 100% finer than 5 microns, 90% finer than 2microns, 70% finer than 0.55 microns, 50% finer than 0.27 microns, 30%finer than 0.16 microns and 10% finer than 0.10 microns was used. Thisclay was obtained from blunged, degritted, fractionated and bleachedcrude kaolin from Georgia. The fine-grade clay was acid flocced byacidifying the slurry to a pH of about 2.5-3.0 with a 10% sulfuric acidsolution to remove dispersant. The suspension was then filtered and thesolids washed with water, air dried at 50° C. overnight andmicropulverized through an 0.020-inch screen prior to use.

The fine-grade clay of average particle size (e.s.d.) of 0.27 micronswas dry-blended with the coarse fraction clay of average particle size(e.s.d.) of 6.4 microns in the ratio 1 part fine to 4 parts coarse byweight. The blend was calcined in a laboratory muffle furnace at 925° C.for 90 minutes then milled in a Lancaster Mixer-Muller for 1 hour andsubsequently micropulverized using an 0.020 inch diameter round-holescreen. The particle size distribution of the resulting product was 100%finer than 44 microns, 90% finer than 13.5 microns, 70% finer than 7.3microns, 50% finer than 4.8 microns, 30% finer than 2.8 microns, 20%finer than 0.9 microns and 10% finer than 0.45 microns. The oilabsorption of the calcined and milled blend was 25 g. oil/100 g. clay byASTM. By way of comparison, the oil absorption of the blend prior tocalcining was 38 g. oil/100 g. clay by ASTM.

The chalking rating was evaluated by using 100% blended pigment in anacrylic latex formulation discussed in the example following:

EXAMPLE 6

Single pigment paints for chalking tests were formulated in a latexsystem at 30% PVC. The CPVC's of the pigments varied but were generallyabove 48%. The latex paint formulations based on 100 gallons of paintfollow:

    ______________________________________                                        30% PVC LATEX PAINT FORMULATION                                                              "Grind"                                                        Component        Weight (lbs.)                                                                            Volume (gal.)                                     ______________________________________                                        Water            25.0       3.0                                               Natrosol ® 250HR                                                                           120.0      14.38                                             cellulosic thickener                                                          in 2% solution                                                                Igepal CO630 wetting agent                                                                     3.0        0.34                                              Tamol ® 731 dispersant in                                                                  2.0        0.26                                              25% solution                                                                  Pigment          190-210    9.0                                               Super Ad-it ® biocide                                                                      1.0        0.12                                              Colloid 581B defoamer                                                                          1.5        0.22                                              ______________________________________                                    

Above ingredients were dispersed for 15 minutes at high speed on aPremier Mill Model 200 Dispersator with a 21/2 inch Cowles blade. Thespeed was then reduced and the following ingredients were added andmixed 10-15 minutes (or until smooth appearance achieved) at reducedspeed:

    ______________________________________                                                       "Let-Down"                                                     Component        Weight (lbs.)                                                                            Volume (gal.)                                     ______________________________________                                        Colloid 581B Defoamer                                                                           1.5        0.22                                             UCAR ® 366 acrylic latex                                                                   370.0      40.88                                             Water and 2% Natrosol                                                                          286.4      32.16                                             250HR thickener                                                                                           100.0                                             ______________________________________                                    

The amount of Natrosol thickener added to the "Let-Down" portion of themix was just sufficient to give 85 Ku (Krebs units) on the Stormerviscometer, an instrument for viscosity measurement widely used by thosein the paint art. For the purposes of generating laboratory-sizesamples, the weight values given above were scaled down to give a samplesize of approximately one pint.

                  TABLE I                                                         ______________________________________                                        CHALKING RATING (ASTM D659-74) OF VARIOUS                                     PIGMENTS IN 30% PVC LATEX FORMULATION                                         AFTER 1500 HOURS EXPOSURE IN UVCON                                            Pigment                     Rating                                            ______________________________________                                        Pigment of Example 1 (coarse calcined kaolin)                                                             7                                                 Pigment of Example 2 (coarse calcined kaolin)                                                             9                                                 Pigment of Example 3 (coarse calcined kaolin)                                                             9                                                 Pigment of Example 4 (coarse calcined kaolin)                                                             9                                                 Pigment of Example 5 (calcined kaolin blend)                                                              9                                                 Chem Carb ® 44 pigment (coarse calcium carbonate)                                                     8                                                 ASP ® 400 clay (coarse hydrous kaolin)                                                                5                                                 ______________________________________                                    

A rating of 10 indicates no chalking. Superior resistance to chalking isexhibited by the pigments of Examples 2-4, which had been calcined tosubstantial dehydration, i.e. kaolin clay that had passed through thecharacteristic endotherm. Kaolin clay which had been partiallydehydrated (Pigment of Example 1) showed improved chalking resistanceover hydrous kaolin clay. Kaolin clay that had been substantiallydehydrated showed the highest resistance to chalking of the samplestested, indeed virtually not chalking at all. Although no difference wasfound relevant to chalking resistance between kaolin clay pigment thathad been calcined at 600° C. and that which had been calcined at 900 C.,the 900° C. calcination temperature is preferred because it yields ahigher brightness pigment. Indeed, calcining at temperatures below 800°C. will generally lead to pigments of unsatisfactory brightness.

EXAMPLE 7

In order to show that pigments of the present invention possess highopacifying power and good optical properties, several paint samples wereformulated at 45% PVC and evaluated for optical properties. Theformulation used is given below:

    ______________________________________                                        45% PVC LATEX PAINT FORMULATION                                                              "Grind"                                                        Component        Weight (lbs.)                                                                            Volume (gal.)                                     ______________________________________                                        Water            26.0       3.12                                              Natrosol HR250 cellulosic                                                                      120.0      14.38                                             thickener in 2% solution                                                      Igepal CO630 wetting agent                                                                     3.0        0.34                                              Tamol 731 dispersant in 25%                                                                    5.0        0.66                                              solution                                                                      TiO.sub.2 Primary Pigment                                                                      225.0      6.76                                              Extender pigment 122-128    5.70                                              Super Ad-it biocide                                                                            1.0        0.12                                              Colloid 581B defoamer                                                                          1.5        0.22                                              ______________________________________                                    

Above ingredients were dispersed for 15 minutes at high speed on aPremier Mill Model 2001 Dispersator with a 21/2 inch Cowles blade. Thespeed was then reduced and the following ingredients were added andmixed 10-15 minutes (or until smooth appearance achieved) at reducedspeed:

    ______________________________________                                                         "Let-Down"                                                   Component          Weight (lbs.)                                                                            Volume (gal.)                                   ______________________________________                                        Colloid 581B defoamer                                                                             1.5       0.22                                            UCAR 366 acrylic latex                                                                           266.4      29.6                                            Water and 2% solution of Natrosol                                                                324.9      38.99                                                                         100.0                                           ______________________________________                                    

Extender pigments included calcined kaolin of the present invention,hydrous kaolin, and calcium carbonate. The amount of Natrosol thickeneradded to the "Let-Down" portion of the mix was just sufficient to give85 Ku (Krebs units) on the Stormer Viscometer. For the purpose ofgenerating laboratory-size samples, the weight of each ingredient wasscaled down to give a sample size of approximately one pint.

Samples of paint were so prepared using various extender pigments toevaluate the optical properties. These are shown in Table II below:

                  TABLE II                                                        ______________________________________                                        OPTICAL PROPERTIES OF VARIOUS EXTENDER                                        PIGMENTS IN FULL FORMULATION, 45% PVC                                         LATEX PAINT                                                                                  Hiding                                                                        Power     % Tinting Percent                                    Extender Pigment                                                                             ft..sup.2 /gal.*                                                                        Strength**                                                                              85° Sheen                           ______________________________________                                        Hydrous Kaolin 332       100       7.7                                        (ASP 400 pigment)                                                             Pigment of Example 4                                                                         339       100       6.7                                        (coarse calcined kaolin)                                                      Pigment of Example 5                                                                         326       100       5.7                                        (calcined kaolin blend)                                                       Chem Carb 44 Pigment                                                                         269        93       6.7                                        (coarse calcium carbonate)                                                    Gold Bond ® R Pigment                                                                    298        97       2.9                                        (coarse silica)                                                               ______________________________________                                         *at 0.98 contrast ratio, determined by KubelkaMunk analysis.                  **relative to ASP 400 hydrous kaolin.                                    

These results show that the desirable optical properties of kaolinpigments are retained by the calcined kaolin pigments of the presentinvention.

What is claimed is:
 1. An exterior grade paint comprising an improvedcalcined kaolin clay pigment and film-forming binder, said improvedcalcined kaolin clay pigment having been obtained by a process whichcomprises:(a) fractionating a hydrous clay to obtain a fine fractionhaving an average particle size less than about 1 micron, e.s.d., andfractionating the same or a different hydrous clay to obtain a coarsefraction having an average size above about 3 microns, e.s.d., (b)blending said fine fraction and said coarse fraction clay such that theresulting blend has an average particle size of at least 3 microns,e.s.d., (c) calcining said blended clay under conditions of time andtemperature such that said hydrous kaolin clay is substantiallydehydrated by passing through the characteristic kaolin endotherm, and(d) milling said calcined blended clay without reducing average particlesize below about 3 microns, e.s.d. until an oil absorption in the rangeof about 10-30 g. oil/100 g. clay as determined by ASTM spatulla ruboutis achieved; said calcined kaolin clay pigment bring present in saidpaint at a reduced pigment volume concentration below 1 and saidfilm-forming binder being an acrylic or vinyl acrylic resin containingglobules 0.1 to 1.0 micron diameter and having a molecular weight of10,000 to 1,000,000.
 2. An exterior grade paint comprising an improvedcalcined kaolin clay pigment and film-forming binder, said improvedkaolin clay pigment having been obtained by a process whichcomprises:(a) blending coarse calcined kaolin clay particles having anaverage particle size above about 3 microns, e.s.d., and fine calcinedkaolin clay particles having an average particle size smaller than about1 micron, e.s.d., the resulting blend having an average particle size ofat least 3 microns, e.s.d., and (b) milling said blended particleswithout reducing average particle size below about 3 microns, e.s.d.until an oil absorption in the range of about 10-30 g. oil/100 g. clayas determined by ASTM spatula rubout is achieved, said calcined kaolinpigment being present in said paint at a reduced pigment volumeconcentration below 1 and said film forming binder being an acrylic orvinyl acrylic resin containing globules 0.1 to 1.0 micron diameter andhaving a molecular weight of 10,000 to 1,000,000.